1. Field of Invention
This invention relates to ink jet printing, and more particularly, to using an electric field to charge and impart a force onto ink drops such that the ink drops are moved toward, and impact upon, a print medium. The invention is also directed to a transfuse printing system that utilizes ink jet printing.
2. Description of Related Art
Conventional ink drop printing systems use various methods to form and impact ink drops upon a print medium. Well-known devices for ink drop printing include thermal ink jet print heads, piezoelectric transducer-type ink jet print heads and bubble jet print heads. Each of these print heads produces approximately spherical ink drops having a 15 to 100 xcexcm diameter. Acoustic ink jets can produce drops that are less than 15 xcexcm in diameter. These smaller ink drops lead to increased resolution. Conventional print heads impart a velocity of approximately four meters per second on the ink drops in a direction toward the print medium.
Actuators in the print heads produce the ink drops. The actuators are controlled by a marking device controller. The marking device controller activates the actuators in conjunction with movement of the print medium relative to the print head. By controlling the activation of the actuator and the print medium movement, the print controller directs the ink drops to impact the print medium in a specific pattern, thus forming a desired image on the print medium.
Conventionally, the actuators also impart an impulsive force to propel the ink drops across a gap separating the print head and the print medium. A significant amount of energy is required to both form and propel the ink drops. Moreover, some types of actuators are very inefficient. For example, the efficiency of piezoelectric devices is approximately 30%. In acoustic ink jet printing, approximately 95% of the energy input to form and expel the ink drops is lost in the form of excess heat. Such excess heat is undesirable because it raises the operating temperature of the surrounding components, such as the print head. This leads to thermal stresses that decrease the long-term reliability of the device.
U.S. patent application Ser. No. 08/480,977 entitled xe2x80x9cElectric-Field Manipulation of Ejected Ink Drops in Printingxe2x80x9d, which is commonly assigned, discloses providing an electric field to assist in directing ink drops toward the print medium in a desired manner, e.g., by selectively deflecting the ink drops slightly to enhance the resolution of the image produced by a given print head configuration. The ink jet actuators form and impart an initial velocity on the ink drops. The charged ink drops are then steered by electrodes such that the drops alternately impact upon the print medium at positions slightly offset from positions directly opposite the apertures of the print head.
Although this method increases the resolution of the image formed on the print medium, it does not address the problem of controlling the operating temperature of the print head. As a result, the high print head operating temperature shortens the usable life of the device.
Further, this method does not address the problem of satellite drops. Satellite drops are formed due to imperfections in the formation of primary ink drops. Satellite drops are much smaller than primary drops, and thus tend to be more influenced by environmental conditions, e.g., air currents in the gap. In conventional devices, the satellite drops decelerate rapidly due to higher air drag. At some point, the satellite drops return and impact on the print head. Other drops that cross the gap produce undesirable printing artifacts due to the result of air currents that reduce the print quality. This result is undesirable because the accumulation of satellite drops on the print head can decrease its performance over time.
Additionally, printing systems are known in which phase-change ink jet images are simultaneously transferred and fused to paper. These printing systems use metal intermediates that have a coating of a sacrificial liquid layer to insure release of the phase-change ink images formed thereon. In these systems, ink is ejected onto the metal intermediate to form ink images which are then transferred from the metal intermediate to paper. The image quality derived from a transfuse process is typically superior to direct marking on paper. However, with conventional ink jet, this quality can be compromised by the fact that drops do not land at the exact desired position on the intermediate, and the fact that the primary drop and any satellite drops do not travel at the same velocity. It is a further disadvantage of transfuse systems that they require significant heating of the paper, and hence require significant energy consumption. At higher print speeds, the combined energy requirements for the print head, ink reservoir and delivery system, and transfuse subsystems, can exceed the typical AC outlet capacity in an office environment. Such printing systems are described, for example, in U.S. Pat. Nos. 5,389,958; 5,372,852; 5,502,476; and 5,614,933.
It would be advantageous to.provide a method and a device for performing ink jet printing at a decreased operating temperature as a result of a lower required energy input.
It would also be advantageous to configure a marking device such that primary ink drops and satellite ink drops impact the print medium at the same time.
It would also be advantageous to control ink drop size such that ink drops having diameters of less than 15 xcexcm are formed.
It would also be advantageous to facilitate biasing of the drops by induction.
These and other advantages are achieved by the method and apparatus of the present invention. The method includes the steps of generating an electric field across a gap between a print head and a print medium in a marking device, forming the ink drops adjacent the print head and controlling the electric field. The electric field is controlled such that an electrical attraction force exerted on the formed ink drops by the electric field is a greatest force acting on the ink drops.
The generating step can include biasing the print support medium with a voltage source. Further, the generating step can include charging the print head, e.g., setting the print head to ground.
The ink drops can be formed by exerting an ink drop forming force slightly greater than a threshold surface tension force that acts in a direction opposite the drop forming force.
The electric field can be controlled to maintain a field strength of approximately 1.0 V/xcexcm. The electric field can also be controlled such that a travel time from the print head to the print medium is approximately the same for the primary and satellite ink drops that are smaller than the primary ink drops. The ink drops can be formed to have a radius of at least approximately 1 xcexcm and not greater than 15 xcexcm.
Forming the ink drops can include producing a plume of ink extending in a direction from the print head toward the print medium and separating an end portion of the plume to form the ink drops.
The electric field can be generated by a voltage source. The drops can be formed by an acoustic ink jet-type actuator. The gap between the print head and the print medium is preferably approximately 1 millimeter.
The apparatus of the present invention includes an ink jet marking device having a print head for forming an image on a print medium. The print head is separated from the print medium by a gap. The marking device includes a generating device that generates an electric field across the gap, a drop forming device that forms drops of ink adjacent the print head and a controller coupled to the drop forming device for controlling the electric field such that an electrical attraction force exerted on the formed ink drops is greater than other forces acting on the ink drops. The drop forming device is coupled to the generating device.
The ink jet marking device can also include a print medium support positioned on a side of the print medium opposite the print head. The print medium support is coupled to the generating device such that the generating device produces a voltage on the print medium support. Preferably, the generating device is a voltage source.
The drop forming device preferably forms drops of ink by exerting a drop forming force slightly greater than a threshold surface tension force acting in an opposite direction. Preferably, the drop forming device includes an acoustic ink jet-type actuator.
The apparatus of the present invention includes an ink jet marking device having a print head for forming an image on a print medium. The print head is separated from the print medium by a gap. Preferably, the marking device includes a generating device that generates an electrical field across the gap, a drop forming device that forms drops of ink adjacent the print head and a controller coupled to the generating device and the drop forming device for controlling the electric field such that an electrical attraction force exerted on the formed ink drops is greater than other forces acting on the ink drops.
The ink jet marking device can also include a print medium support positioned on the side of the print medium opposite the print head. The print medium support is coupled to the generating device such that the generating device produces a voltage on the print medium support. Preferably, the generating means is a voltage source.
The drop forming means preferably forms drops of ink by exerting a drop forming force slightly greater than a threshold surface tension force acting in an opposite direction. Preferably, the drop forming device includes an acoustic ink jet-type actuator.
This invention also provides a transfuse printing system in which the ink jet marking device is used to create an ink image on an intermediate medium which is subsequently transferred and fused to a final print medium. The transfuse printing system includes a belt as the intermediate medium and grounded rollers that serve to limit an area of high voltage to an area under the ink jet marking device.
The belt may be constructed of one or more materials that facilitate dissipation of electrical charge from the incident ink drops through the thickness of the belt and lateral voltage dissipation to the grounded rollers without excessive current to the grounded rollers. The dissipation of electrical charge through the thickness of the belt eliminates the negative effects of charge build up and image blooming. These materials may include materials having an intermediate conductivity such that the belt has a controlled conductivity that allows electrical charge to be dissipated through the thickness of the belt and voltage to be laterally dissipated to the grounded rollers.
Additionally, the ink jet marking device allows a thin layer of ink to be ejected onto the belt and thus, thermal build-up is reduced due to the reduction in the ink layer thickness.